This invention relates to the preparation of polysaccharides, particularly, to the preparation of cross-linked, etherified polysaccharides.
Polysaccharide derivatives, particularly water-soluble, etherified polysaccharides such as methylcellulose, are used extensively as binders, film formers and thickeners in various products including cosmetics, adhesives, printing inks and paints. Conventionally, the polysaccharide ethers are prepared by reacting the reaction product of a polysaccharide and an alkali metal hydroxide with an etherifying agent, e.g., methyl chloride or butylene oxide, in the presence of an inert, reaction diluent such as toluene.
In general, when employed as thickeners, the etherified polysaccharides advantageously exhibit a relatively high viscosity in an aqueous liquid. This ability of the etherified polysaccharide to effectively thicken an aqueous liquid is primarily controlled by the molecular weight (degree of polymerization) of the polysaccharide, with the higher molecular weight polysaccharides generally exhibiting the higher viscosities. Unfortunately, the molecular weight of etherified polysaccharides is limited by the molecular weight of the polysaccharide starting material and the extent of the degradation of the polymer structure during the etherification reaction.
Heretofore, to increase the molecular weight of polysaccharide derivatives, they have been cross-linked with various cross-linking agents. For example, U.S. Pat. No. 2,650,917 teaches that a polysaccharide derivative such as a derivative of glucomannan gum can be cross-linked with a polyfunctional etherification reagent such as bis-(2-chloroethyl ether) or epihalohydrin to render a cross-linked product exhibiting exceptional higher viscosities upon dissolution in water. Other cross-linking agents disclosed as being useful in preparing cross-linked polysaccharides and polysaccharide derivatives which exhibit improved viscosities in water include dichlorobutene and dichloroacetic acid. See, for example, U.S. Pat. Nos. 2,988,455 and 3,152,925. While the prior art cross-linking agents effectively cross-link the polysaccharides and derivatives thereof, thereby increasing their molecular weights, the concentration of the cross-linking agent and the conditions of the cross-linking reaction must be carefully controlled to prevent the formation of relatively large amounts of unwanted, water-insoluble materials. Moreover, most of the disclosed cross-linking agents are highly reactive and cannot be handled without risk unless special precautions are taken.
In view of the stated deficiencies of the prior art, it remains highly desirable to prepare water-soluble, cross-linked polysaccharide ethers which exhibit increased viscosities when dissolved in water without the coincident formation of substantial amounts of water-insoluble product.